Animal manure and other related sewage slurries from animal farms need to be treated before being released to the land field to prevent the contamination of the underground water system and the spreading of diseases related to bacteria contained in the manure slurry. There exist two competitive types of processes for the treatment of manure slurry. The first type of process treats the slurry biologically in which the waste solution is digested via aerobic or anaerobic fermentation, then the treated slurry which has a water content around 80% is either applied to the land field, or further treated to reduce water content to about 10% and then used as fertilizers. The other type of process uses physical operations to treat the slurry in which the slurry is dried by direct or indirect contact with hot air or gas stream to evaporate the water, the generated solids have a moisture content of about 10% and can be used as fertilizers. The generated gas-vapor mixture is further treated to remove organic compounds and odor gases via incineration or wet scrubbing before it is released to the environment. In recent years, the amount of biologically digested sludge to be spread on the land field has been greatly limited due to the more strict regulations on environmental protection. Most of the digested slurry thus needs to be treated more thoroughly to remove naturally undegradable pollutants or to remove water to such a level that the solids sludge can be used as substitute fertilizers. As a result, the direct drying and granulation process becomes economically advantageous over the biological treatment process.
There exist several possible processes for the drying of animal manure slurry, such as the spray drying process described by Masters, K. (1972), "Spray drying", Leonard Hill Books, London; rotary drying process described by Glorioso (1993) in Canadian Patent 1,325,583; traveling grate drying process described by Bernes and Jensen (1991) in Canadian Patent 2,085,583; and fluidized bed dryer as described by Huttlin (1991) in Canadian Patent 2,000,299. To reduce the sludge water content from 90% to less than 10%, extensive evaporation is involved which requires high heat and mass transfer rates. Compared to rotary dryers and traveling grant dryers, a fluidized bed in which slurry is sprayed on the bed material and evaporation occurs on the surface of particles can provide much larger surface area for the heat and mass transfer, leading to much higher overall mass and heat transfer rates. The gas-solids contact can be further improved in a spout-fluid bed due to the high internal solids circulation inside the spout-fluid bed (Uemaki, O. and Tsuji, T., "Particle velocity and solids circulation rate in a jet-spouted bed", Can. J. Chem. Eng., 70, 925-928, 1992). As a result, spout-fluid bed dryers can provide a drying efficiency more than 10 times higher than spray dryers (Masters 1972; Markowski, A. and Kaminski, W., "Hydrodynamic characteristics of jet-spouted beds", Can. J. Chem. Eng., 61, 377-3811, 1983).